The invention relates to magnetic transducers using the magnetostrictive effect for creating an easy axis of permanent magnetization in a material. This magnetization is able to change under the action of an external magnetic field to be detected.
A magnetic transducer of the type briefly described hereinbefore has already been described. This transducer incorporates a wire formed from a special nickel-iron alloy (permalloy). This wire undergoes a complex treatment giving it special properties as regards to its magnetization and the wire can then be used as a magnetic transducer. This treatment consists of drawing the wire and then twisting it a certain number of times, whilst fixing one end. The other end is then turned in one direction and then in the other direction. At the end of this treatment, it is found that when the wire is placed within a coil and a magnetic field acts on the wire, a pulse can be detected at the coil terminals. This pulse is due to the change in the magnetization created within the wire by the converse magnetostrictive effect as a result of strains applied to the wire. As a result of the treatment applied to the wire, a strong coercive force h.sub.c is created in the area where the wire was strained, i.e. on the wire periphery. The coercive field h.sub.c in the wire core is much lower. When a magnetic polarization field is applied to the wire, magnetization is created which remains after removing the polarization field. Thus, a demagnetizing field is created which, if it is higher than the coercive field in the core of the material, but lower than the coercive field on the periphery of the material, leads to a resultant magnetization whose direction is different in the interior and on the periphery of the wire. When such a wire is placed in a coil and exposed to an external magnetic field which is higher than the demagnetizing field, the core magnetization is changed. When the external field disappears, the magnetization changes back in the other direction.
In such a transducer, only part of the material contributes to the variations in the magnetic flux during the change of the magnetization, due to the fact that the magnetization of the material periphery is already the same as that resulting from the change.
Moreover, the amplitude of the pulses detected is relatively low, approximately 1 volt when the input impedance of the detection system is 10 k.OMEGA.. In addition, this amplitude varies significantly from one change to the next, it being a function of the magnetization quantity subject to the change, which is itself a function, inter alia, of the number of regions which automatically re-changed at the end of the previous phase. Finally, the pulse rise time is relatively long and approximately 100 .mu.s. When such a transducer is used as a position transducer, for example with the object of measuring the rotational speed of a motor, it is necessary that the motor blades do not pass in the vicinity of the transducer at times which are too close together. Thus, such a device, taking account of the pulse rise time s obtained, does not make it possible to measure rapid speeds. Moreover, as the amplitude of the detected pulses is relatively low it is absolutely indispensible to amplify them before using them for example in a counting circuit, which makes additional processing necessary.